Youngsup Song

Impaired macrophage autophagy induces systemic insulin resistance in obesity.

Obesity-induced insulin resistance and diabetes are significantly associated with infiltrates of inflammatory cells in adipose tissue. Previous studies recognized the involvement of autophagy in the regulation of metabolism in multiple tissues, including β-cells, hepatocytes, myocytes, and adipocytes. However, despite the importance of macrophages in obesity-induced insulin resistance, the role of macrophage autophagy in regulating insulin sensitivity is seldom addressed. In the present study, we show that macrophage autophagy is important for the regulation of systemic insulin sensitivity. We found that macrophage autophagy is downregulated by both acute and chronic inflammatory stimuli, and blockade of autophagy significantly increased accumulation of reactive oxygen species (ROS) in macrophages. Macrophage-specific Atg7 knockout mice displayed a shift in the proportion to pro-inflammatory M1 macrophages and impairment of insulin sensitivity and glucose homeostasis under high-fat diet conditions. Furthermore, inhibition of ROS in macrophages with antioxidant recovered adipocyte insulin sensitivity. Our results provide evidence of the underlying mechanism of how macrophage autophagy regulates inflammation and insulin sensitivity. We anticipate our findings will serve as a basis for development of therapeutics for inflammatory diseases, including diabetes.

Beclin‐1 Is Required for RANKL‐Induced Osteoclast Differentiation

Beclin‐1 plays a critical role in autophagy; however, it also contributes to other biological processes in a non‐autophagic manner. Although studies have examined the non‐autophagic role of autophagy proteins in the secretory function of osteoclasts (OC), the role of Beclin‐1 is unclear. Here, we examined the role of Beclin‐1 in OC differentiation, and found that mouse bone marrow macrophages (BMMs) showed increased expression of Beclin‐1 upon RANKL stimulation in a p38‐ and NF‐kappa B‐dependent manner. During OC differentiation, Beclin‐1 localized to the mitochondria, where it was involved in the production of mitochondrial intracellular reactive oxygen species. Knockdown of Beclin‐1 in RANKL‐primed BMMs led to a significant reduction in RANKL‐dependent osteoclastogenesis, which was accompanied by reduced NFATc1 induction. Furthermore, knockdown of Beclin‐1 inhibited RANKL‐mediated activation of JNK and p38, both of which act downstream of reactive oxygen species, resulting in the suppression of NFATc1 induction. Finally, overexpression of constitutively active NFATc1 rescued the phenotype induced by Beclin‐1 knockdown, indicating that Beclin‐1 mediates RANKL‐induced osteoclastogenesis by regulating NFATc1 expression. These findings show that Beclin‐1 plays a non‐autophagic role in RANKL‐induced osteoclastogenesis by inducing the production of reactive oxygen species and NFATc1. J. Cell. Physiol. 229: 1963–1971, 2014.

PTX3 Stimulates Osteoclastogenesis by Increasing Osteoblast RANKL Production

Pentraxin‐3 (PTX3), also known as tumor necrosis factor‐stimulated gene 14 (TSG‐14), is produced by immune and vascular cells in response to pro‐inflammatory signals and is therefore a multipotent inflammatory mediator. The present study showed that during human osteoblast (OB) differentiation, precursor OBs (pOBs), but not mature OB, highly expressed PTX3. TNFα treatment elevated the PTX3 expression of pOBs. When mice were injected with lipopolysaccharide, which induces an inflammatory osteolytic condition characterized by trabecular bone destruction and high osteoclastogenesis, their bone marrow cells expressed elevated levels of PTX3 protein. Exogenous PTX3 did not directly affect osteoclast (OC) or OB differentiation. However, when pOBs and precursor OCs were co‐cultured, exogenous PTX3 significantly increased the number of tartrate‐resistant acid phosphatase‐positive multinucleated cells (i.e., OC cells) by increasing the pOB mRNA expression and protein secretion of RANK ligand (RANKL). This was accompanied with increased Runt‐related transcription factor 2 (Runx2) expression in the pOBs. Knock‐down of endogenous PTX3 with small‐interfering RNA did not change the osteogenic potential of pOBs but suppressed their production of RANKL and reduced osteoclastogenesis. Finally, TNFα treatment of the co‐culture elevated PTX3 expression by the pOBs and increased OC formation. This effect was suppressed by PTX3 knock‐down by decreasing RANKL expression. Thus, the PTX3‐driven increase in the osteoclastogenic potential of pOBs appears to be mediated by the effect of PTX3 on pOB RANKL production. These findings suggest that PTX3 is an inflammatory mediator that contributes to the deteriorating osteolytic condition of inflamed bone. J. Cell. Physiol. 229: 1744–1752, 2014.

Interleukin-1β promotes the LC3-mediated secretory function of osteoclast precursors by stimulating the Ca2+-dependent activation of ERK

Bone resorption by osteoclasts requires the release of secretory lysosomes containing cathepsin K, which degrades the organic bone matrix. The activity of this secretory function is determined by the level of lipidation of microtubule-associated protein 1 light chain 3 (LC3). Although the inflammatory cytokine IL-1β increases osteoclast activity, the underlying mechanism(s) remains undefined. In our present study, we found that IL-1β accelerates the release of cathepsin K from osteoclast precursors by increasing the cleavage and lipidation of LC3 and the subsequent formation of lipid-bound LC3-II containing secretory lysosomes. IL-1β increased LC3-II formation within osteoclast precursors through a process that is dependent on increases in the intracellular Ca(2+) levels. In addition, IL-1β was found to act synergistically with RANKL to increase ERK activation in a Ca(2+)-dependent manner. More importantly, Atg7-deficient osteoclast precursors, which showed impaired lipidation of LC3-I, did not exhibit IL-1β-mediated increases in cathepsin K secretion. Thus, IL-1β promotes LC3-II formation through the Ca(2+)-dependent activation of ERK, which triggers the release of cathepsin K. These findings provide evidence for a molecular mechanism through which IL-1β enhances the secretory function of osteoclast precursors.

Panax red ginseng extract regulates energy expenditures by modulating PKA dependent lipid mobilization in adipose tissue.

Regulation of balance between lipid accumulation and energy consumption is a critical step for the maintenance of energy homeostasis. Here, we show that Panax red ginseng extract treatments increased energy expenditures and prevented mice from diet induced obesity. Panax red ginseng extracts strongly activated Hormone Specific Lipase (HSL) via Protein Kinase A (PKA). Since activation of HSL induces lipolysis in WAT and fatty acid oxidation in brown adipose tissue (BAT), these results suggest that Panax red ginseng extracts reduce HFD induced obesity by regulating lipid mobilization.

Zebrafish ghrelin is expressed in pancreatic endocrine cells and regulated by metabolic state.

Mammalian ghrelin is a stomach-derived peptide that stimulates secretion of growth hormone and food intake. Zebrafish is an excellent model system for forward genetic studies, and many aspects of energy homeostasis characterized in mammals appear to be conserved in the zebrafish. In this study, we investigated the expression and regulation of zebrafish ghrelin by metabolic status. Quantitative RT-PCR revealed that zebrafish ghrelin is highly enriched in anterior gut associated tissues. Using in situ hybridization with adult zebrafish tissues, we found that zebrafish ghrelin mRNA was not expressed in intestine tissue, but rather in clusters of endocrine pancreas cells distinct from insulin-expressing islets. Fasting specifically upregulated pancreatic ghrelin but not brain ghrelin expression by 3- to 4-fold and refeeding restored ghrelin transcript to control levels seen in the fed group within 5 h. These results demonstrate that although ghrelin is expressed in a different site in zebrafish, it is responsive to metabolic state in a similar manner as mammalian ghrelin, suggesting a role in the regulation of feeding in teleosts, and thus validate the utility of zebrafish as a genetic model system for the analysis of the ghrelin system and energy homeostasis.

Vancomycin blocks autophagy and induces interleukin-1β release in macrophages

Systemic inflammatory response syndrome (SIRS) is a serious condition that can cause organ failure as an exaggerated immunoresponse to the infection or other causes. Recently, autophagy was reported as a key process that regulates inflammatory responses in macrophages. Vancomycin is one of the most commonly prescribed antibiotics for sepsis treatment or following surgery. However, there are no studies on how vancomycin affects autophagy or inflammation. Here, we treated macrophage cell lines with vancomycin and lipopolysaccharides and found that vancomycin blocks autophagy and increases inflammatory responses. This finding suggests that vancomycin should be more cautiously administered in order to prevent unwanted SIRS during sepsis.

Molecular cloning, regulation, and functional analysis of two GHS-R genes in zebrafish.

Mammalian ghrelin is derived from stomach and regulates growth hormone release and appetite by modulating GHS-R (Growth hormone secretagogue receptor) activity. Zebrafish has been developed as a forward genetic screening model system and previous screening identified a number of genes involved in multiple signaling pathways. In this system, ghrelin has been identified and its function and regulation have been shown to be highly conserved to that of mammals. Here, we identified three isoforms of zGHS-R1 and one of zGHS-R2 (zGHS-R2a), and characterized their expression, regulation and function. Three isoforms of zGHS-R1, which we named zGHS-R1a, zGHS-R1b, and zGHS-R1c, are generated by alternative splicing. The expression of zGHS-R1 is highly enriched in brain, intestine tissue, and skin tissues. Compared to zGHS-R1, the expression pattern of zGHS-R2a is rather evenly distributed. A 15 day fasting elevated expression of zGHS-R1 and zGHS-R2 transcripts in anterior intestine tissues, but not in brain. Whereas zGHS-R1a, zGHS-R1c, and zGHS-R2a appear to be presented on the plasma membrane, the localization of zGHS-R1b seems to be restricted in the intracellular region. Treatment of ghrelin agonist, L692,585 or goldfish ghrelin peptides but not rat ghrelin, elevated intracellular Ca(2+) level and phosphorylation of ERK in HEK-293 cells expressing zGHS-R1a, but not zGHS-R1b, zGHS-R1c, or zGHS-R2a. It appears that besides core ghrelin peptide sequence of GS/TSF additional amino acids are required for the activation of zGHS-R1a, as rat ghrelin induces neither intracellular Ca(2+) mobilization nor ERK phosphrylation. These results suggest that ghrelin system in zebrafish is highly conserved to that of mammals, and thus is an ideal in vivo model for dissecting ghrelin system.

Secretory clusterin inhibits osteoclastogenesis by attenuating M-CSF-dependent osteoclast precursor cell proliferation

Secretory clusterin (sCLU)/apolipoprotein J is a multifunctional glycoprotein that is ubiquitously expressed in various tissues. Reduced sCLU in the joints of patients with bone erosive disease is associated with disease activity; however, its exact role has yet to be elucidated. Here, we report that CLU is expressed and secreted during osteoclastogenesis in mouse bone marrow-derived macrophages (BMMs) that are treated with receptor activator of nuclear factor kappa-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). CLU-deficient BMMs obtained from CLU(-/-) mice exhibited no significant alterations in OC differentiation in comparison with BMMs obtained from wild-type mice. In contrast, exogenous sCLU treatment significantly inhibited OC formation in both BMMs and OC precursor cultures. The inhibitory effect of sCLU was more prominent in BMMs than OC precursor cultures. Interestingly, treating BMMs with sCLU decreased the proliferative effects elicited by M-CSF and suppressed M-CSF-induced ERK activation of OC precursor cells without causing apoptotic cell death. This study provides the first evidence that sCLU reduces OC formation by inhibiting the actions of M-CSF, thereby suggesting its protective role in bone erosion.

Low expression level of OB-Rb results from constitutive translocational attenuation attributable to a less efficient signal sequence

OB‐Rb is a crucial factor for leptin signaling. This study was initially motivated by the observation that OB‐Rb expression is constitutively inhibited in the early secretory pathway. Our analyses reveal that OB‐Rb contains a less hydrophobic, but functionally active N‐terminal signal sequence. Constitutive translocational attenuation attributable to a less efficient signal sequence proved to be a reason for low protein level of OB‐Rb. By contrast, enhanced signal sequence efficiency rescues translocation and cell surface expression of OB‐Rb, and eventually potentiates leptin signaling. These observations provide considerable insight into the therapeutic enhancement of OB‐Rb translocation as a potential strategy for leptin resistance.